14 Corriente pulsanteThe most important weld parameters are:pulse current lpbackground current lGpulse current time tpbackground current time tGpulse frequency fp = 1 / tcWhere: tc = duration of period

15 Corriente pulsanteConcerning the equipment of TIG-pulsed arc welding, a relatively new pulsed arc welding process has emerged, which only modifies by the current (pulse amplitude, impulse frequency, mark-space ratio).During the impulses where high current is present in the pulse arc process, a large amount of heat is generated in the welding area. This results in fusion of the work material.In the impulse pause where low current is preset, only a little heat is transmitted into the workpiece, thus the weld pool stays comparatively cool. The low currents during the background current time only serve to maintain the arc in order to avoid interruptions and ignition difficulties. When welding with a filler, the filler will be fused with the base material during the impulse phase. The impulse frequency is usually between 0.5 Hz and 10 Hz.

16 Corriente pulsanteThe weld heat input can be considerably changed by the choice of times and current values. In the extreme case a weld seam can consist of fusion welding points which lie next to each other or overlap.Thanks to the TIG-pulsed arc welding, the area of application of the TIG-process can be extended to low power values thus material thicknesses can be reduced and the weld seam appearance can again be improved.For welding aluminium with a DC supply it is only possible to use helium as shielding gas.

20 Destruction of the Oxide LayerCorriente alternaDestruction of the Oxide LayerIn the case with the electrode as cathode, the emitted electrons meet the anodic poled workpiece and, by the conversion of kinetic energy, they generate a large amount of heat on the point of contact and thus give a deep penetration. In comparison, the electrode tip is only heating up a small amount, due to the upcoming gas ions, which in contrast to the electrons show a larger mass, but generate less heat and are considerably slower than the electrons. Operating using this polarity the oxide layer is not destroyed, which in reality means that this polarity is not suitable for aluminium welding.In the case with the electrode as anode, the emitted electrons meet the electrode and heat it up rapidly. In comparison, the workpiece which is poled as a cathode is only heated a relatively small amount, thus only a flat penetration arises.

21 Destruction of the Oxide LayerCorriente alternaDestruction of the Oxide LayerAt this polarity a cleaning effect comes up, i.e. the oxide skin is torn apart and detached. This effect is explained by the fact that the relatively heavy ions meet the oxide skin and destroy it. At this polarity however, the high thermal load on the tungsten electrode leads to a rapid destruction of the tungsten.By using this kind of polarity several welding procedures can carried out by using disproportionately thick tungsten electrodes for thin plates. However, this kind of polarisation is not generally used for TIG welding.

22 Corriente alternaTIG welding with an alternating current is mostly used for practical fabrication.During the positive half wave a cleaning effect occurs and the tungsten electrode rapidly heats up; during the negative half wave the electrode is allowed to cool down.Consequently, the advantages of both kinds of direct current polarity are united. Since the arc goes out at every current zero crossing, it was traditionally supplied by a high frequency overlay (150 kHz at 1500 to 2000 Amps) in order to facilitate a re-ignition of the arc.These machines have now been replaced by pulse generators that do not constantly send out high frequency stress impulses, but instead supply a smooth sinusoidal voltage. This has the advantage that they are far less like to influence radio and TV receptions in the close environment and, as a consequence, do not have to be signed up at the federal post office.

32 ElectrodosWThe pure tungsten electrode is the one without any addition of oxide. It has a low current capacity and is easily burnt. It is suitable for application under the condition of AC and in the situation of low welding requirement

33 ElectrodosZr-WZirconiated Tungsten Electrodes have good performance in AC welding. Especially under high load current its excellent performance can not be replaced by any other products.

34 ElectrodosTh-WThoriated tungsten electrode is a good general use tungsten for DC applications, because it operates well even when overlload with extra amperage, thus improves the performance of welding.

35 ElectrodosLa-WThe lanthanated tungsten electrode has outstanding welding performance without radiation hazard. And its electric conductivity is the most close to that of 2% thoriated tungsten electrode. It enable welders to replace the thoriated tungsten electrode by lanthanated tungsten electrode easily and conveniently without any change in welding procedure. Therefore, lanthanated tungsten electrode is the most popular replacement of 2% thoriated tungsten in Europe and Japan.Lanthanated tungsten electrode is normally applied in DC (Direct Current) welding, it also works well in AC (Alternate Current) welding.

38 Características de la punta del electrodoElectrodosCaracterísticas de la punta del electrodoElectrode taper is usually called out in degrees of included angle, usually anywhere between 14 and 60 degrees. Grinding an electrode to a point aids arc starting when depositing short-duration welds on small parts. However, in most cases a flat spot or tip diameter at the end of electrode works best.

45 8.5. Gases protectoresIn the case of aluminium, the argon offers a calm and stable metal transfer. However it has a lower penetration intensity and its security against porosity (due to hydrogen) is not as resistant as argon-helium mixtures.Helium is not an appropriate shielding gas because of its very uneven coarse drops and often with background current burdened metal transfer.Effective combinations of helium and argon have been found to lie between 30-70% of each respective gas. Most commonly used is a mixture of 50% argon and 50% helium.The pre-heat expenditure can be reduced or totally avoided by using helium-bearing mixtures.For increasing the weld penetration it is possible to add in O2 or CO2 between 150 and 300 vpm instead of N2.

46 8.5. Gases protectores ArgonConcerning TIG-welding with negative polarisation of the electrode, a method has been developed which, instead of the usual inert-gas argon, makes use of the helium gas. This is based on special characteristics of this gas.Due to the higher ionisation energy of helium compared to argon, a greater welding voltage (approximately 75% greater) provides the same amount of amperage and this leads to a higher thermal input into the workpiece. The higher heat conductivity of helium is another advantage compared to the argon.Because of its lower conductivity of electricity, a disadvantage of helium is the production of a turbulent arc and difficult arc ignition when TIG-welding.

47 8.5. Gases protectores ArgonIn a lot of cases, mixtures of argon and helium result in a usable compromise. From an economical point of view it also has to be considered that helium is more expensive than argon. In addition, due to its lower specific weight, comparatively more helium than argon has to be used for gas shielding purposes.The higher energy input by helium results in an increased welding speed, lower pre-heat temperatures at the same penetration, and a lower tendency for porosity by a hotter weld pool with lower viscosity and better degasification possibilities.TIG-welding of aluminium workpieces with an increasing usage of helium will be introduced in future, particularly at machinery welding.

48 HeliumThe shape of the arc is also mainly influenced by the type of inert gas used. This is predominately due to the physical characteristics of the gas and respective thermal conductivities, also the dissociation of the active gases have an influence.The figure shows the influence of the inert gas on the penetration profile on plate TIG welds in aluminium using different shielding gases.+300 vpm NO+70 vpm N2Argon 4.6Argon Helium

56 (a) General view of and (b) cross-section of a torch used in oxyacetylene welding. The acetylene valve is opened first; the gas is lit with a spark lighter or a pilot light; then the oxygen valve is opened and the flame adjusted. (c) Basic equipment used in oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left-handed, whereas those for oxygen are right-handed. Oxygen regulators are usually painted green, acetylene regulators red.

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